Auxiliary power units (APU)s are devices on vehicles such as aircraft that provide an auxiliary source of electrical power in addition to or in absence of ground power (e.g., when an aircraft is at a gate or parked) or main engine generators (e.g., during taxi or in flight). Generally, APUs are found on commercial aircraft, and larger land and water vehicles such as tanks, trucks, and ships. An aircraft APU for running aircraft electrical systems may generally produce a 28V DC current or a 400 Hz AC current at a voltage of 115V.